Final answer:
When a car is moving backward and has negative acceleration, its speed is decreasing as the acceleration is in the opposite direction of its backward motion.
Step-by-step explanation:
If a car is moving backward and has negative acceleration, this indicates that the direction of the acceleration is opposite to the direction in which the car is considered to be moving backward (which is to say, forward relative to its current backward motion). When the acceleration is in the opposite direction of the motion, such as this case, the speed of the car is decreasing. It can be counterintuitive because, while moving backward, negative acceleration actually reduces the backward velocity, making the car slow down in its backward movement.